docs: add contributing guide

Co-Authored-By: Claude <noreply@anthropic.com>

CONTRIBUTING.md

@@ -0,0 +1,41 @@
+# Contributing
+
+## Code
+
+- Follow the [coding conventions](docs/dev/CONVENTIONS.md): naming, no comments except [technical debt](docs/dev/TechnicalDebt.md), Conventional Commits.
+- Every function either does exactly what it is supposed to or throws. Unimplemented exports call `NotImplemented_nid_no_patch(__func__)` (see [libSceAudioIn](core/libs/prx/libSceAudioIn/Export.cpp)).
+- A silent stub is allowed only when it unblocks a title and only affects the UI; add it to [silent stubs](docs/dev/TechnicalDebt.md#silent-stubs).
+- Missing imports fail at startup with a clear error; don't replace them with fallbacks that keep running.
+- Implement the general behaviour of a function, not what one title happens to need.
+- Avoid non-standard extensions (`__attribute__`, etc.) where standard C++ is enough. Helper symbols that must not become NIDs use the `_nid_no_patch` or `_nid_no_patch_cut` suffix.
+- The relinker uses only the C++20 standard library.
+- Third-party code is added as a submodule under `3rdparty/` and built from source, not found on the system.
+- Don't add tests that only check that a symbol is exported: a missing export already fails at startup.
+
+## Build and test
+
+Toolchains are listed in the [README](README.md#build).
+
+```
+cmake -S . -B build -G Ninja -DCMAKE_BUILD_TYPE=Release -DBUILD_TESTING=ON
+cmake --build build
+ctest --test-dir build --output-on-failure
+```
+
+Python 3 is optional; without it some relinker tests are not registered.
+
+## Branches and pull requests
+
+The [pull request template](.github/pull_request_template.md) is the checklist for these rules.
+
+- One branch per topic, based on current `main`. Follow-up work goes in a new pull request, not into an open one.
+- Before starting, check that no open pull request already implements the same functions.
+- Keep the branch up to date with `main` and resolve conflicts yourself; rebasing and force-pushing is fine.
+- If a pull request needs another one first, say so in the description (`Depends on #N`).
+- Run the tests before opening the pull request and describe what was tested (OS, title or homebrew).
+- Investigation notes, reports, screenshots and logs go in the pull request, not in the repository. Images for documentation go in the [gist](https://gist.github.com/boykopovar/0e53f2e1426f29ecd41e3b51540b8a90) comments.
+- Say whether the change was written with AI assistance. The author of the pull request is responsible for every line of it.
+
+## Documentation
+
+Text in the repository is dry, strict and concise. Record known gaps, unknown NIDs and signatures in [technical debt](docs/dev/TechnicalDebt.md) instead of new files.

README.md

@@ -22,7 +22,7 @@ Unsupported or unexpected states strictly throw `std::runtime_error`. `what()` i
 
 The [shader recompiler](core/shader/recompiler/Recompiler.cpp) successfully produces SPIR-V (validated via [Spirv-Tools](3rdparty/SPIRV-Tools) when built with `ANYPS5_ENABLE_SPIRV_TOOLS`).
 
-[Technical debt of the project](docs/dev/TechnicalDebt.md), [code style conventions](docs/dev/CONVENTIONS.md)
+[Technical debt of the project](docs/dev/TechnicalDebt.md), [code style conventions](docs/dev/CONVENTIONS.md), [contributing](CONTRIBUTING.md)
 
 ## Build